Your dealer is making excuses. While it's not exactly something one would do for fun and relaxation, changing out a wheel for a spare on a typical class C is very much doable by ordinary persons. The lug nuts on mine are tightened to 140 ft-lbs, which is well within what one can muscle with something like a two foot breaker bar. The wheels are heavy and unwieldy, but not impossible to move around (and they can be rolled).
I have the spare mounted on a cable lift mechanism underneath the motorhome, and that works out nicely if you have a spot where it can go. Some pickup trucks use a similar sort of a mechanism. (If such a lift were too close to the back of the motorhome, you'd run the risk of it being the first thing to scrape when you encounter a steep transition in the pavement such as at some gas stations.)